move gep decomposition out of ValueTracking into BasicAA. The form of

move gep decomposition out of ValueTracking into BasicAA.  The form of
decomposition that it is doing is very basicaa specific and is only used
by basicaa.

Now with less tree breakingness.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@111433 91177308-0d34-0410-b5e6-96231b3b80d8